feat(shaders): add RadialProgress shader for circular progress / countdown UIs

[chiefcll]

Summary

• New RadialProgress shader — a stroked ring filled to a progress value, with a gradient swept along the arc, an optional background trackColor, and round or butt end caps. Single-node, one draw call, animatable via progress: 0..1.
• Implemented for both WebGL and Canvas2D backends, registered alongside LinearGradient / RadialGradient in exports/{webgl,canvas}-shaders.ts and examples/common/installShaders.ts.
• Companion example at examples/tests/shader-radial-progress.ts showing six variants (defaults, screenshot-recipe countdown, multi-stop gradient sweep, CCW butt caps, full-circle rainbow, custom start angle).

API

```ts
renderer.createShader('RadialProgress', {
width: 14,
progress: 0.7, // 0..1, animatable
startAngle: -Math.PI / 2, // 12 o'clock
direction: 1, // 1 = CW, -1 = CCW
colors: [0x4aa3ffff], // 1..N RGBA swept along the arc
trackColor: 0x1f3a5cff, // 0x00000000 = disabled
cap: 1, // 0 = butt, 1 = round
});
```

Notes for reviewers

• Why a new shader vs. extending RadialGradient? RadialGradient interpolates by distance from a center (filled disc/ellipse). The countdown ring needs interpolation by angle along a stroked arc — different primitive, different fragment shader. Sibling rather than replacement keeps existing RadialGradient consumers untouched.
• WebGL composition: fragment outputs vec4(mix(base.rgb, layer.rgb, layer.a), base.a + layer.a*(1-base.a)) rather than vec4(blended, base.a) — this lets the ring render visibly on a node whose base color alpha is 0 (transparent background), which is the common case for a standalone progress component.
• Canvas2D sweep approximation: Canvas2D has no native sweep gradient, so the arc is drawn as 64 short stroked segments each tinted by interpolating between colors/stops. Segments overlap by 2% to avoid AA seams.
• Cache markers: colors:N|cap:0|1|track:0|1 so shader programs are reused across compatible prop variations.
• Round cap discs are gated to render only when `0 < progress < 1` (no caps at empty or full ring).

Test plan

• `pnpm build` — clean
• `pnpm exec vitest run` — 182/182 pass (14 new for the template's resolvers)
• Visual sanity in `pnpm start` (WebGL): six rings render correctly
• `pnpm test:visual:update` — generate and review the certified snapshot for `shader-radial-progress` before merging
• Canvas2D path manually verified in `pnpm start` with `?renderMode=canvas`
• `pnpm start:prod` to confirm transpile-floor compatibility (Chrome 38)

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